The present invention relates generally to gain adjustment circuits for an amplifier and, more particularly, relates to a single or common adjustable control mechanism for an amplifier, which has multiplexed feedback channels to provide the amplifier with different respective gains, to effect for each channel simultaneous linear proportional changes in the effective gains corresponding fractional amounts of the respective ranges of permissible gain change for each.
The gain of a typical feedback stabilized amplifier is normally a function of the impedance ratios in the feedback circuit or channel between the inverting input and output thereof. For purposes of this application the impedances coupled in an amplifier circuit will be considered hereinafter as resistances; however, it will be appreciated that other impedances may be employed equivalently within the spirit and scope of the invention. To change the amplifier gain, then, the ratio of the resistances in the feedback channel may be simply changed.
Moreover, a single amplifier may have a plurality of feedback channels of different respective impedances selectively connected between the amplifier input and output, for example, by conventional multiplexing techniques, selectively to provide the amplifier with different respective basic gains. This multiplexing technique is especially useful when a single amplifier is employed to amplify with different respective gains one or more input signals received on one or more input channels to the amplifier. However, each of those respective feedback channels normally would require separate initial calibration and usually would require subsequent separate calibrations to compensate for circuit fatigue or drift or the like experienced in the input channel or channels, for example, due to changing temperatures, aging, etc. It is, of course, desirable to reduce the amount of time necessary to perform these calibrations in order to increase the available operative time of an instrument employing such an amplifier arrangement.
As a non-limiting example of an instrument employing such a multiplexed amplifier arrangement, a colorimeter may use a single photosensor to detect red, green, and blue light. The electrical output from the sensor for a given intensity of red light impinging thereon may actually be as much or more than ten times the electrical output therefrom for an equal intensity of blue light. Therefore, the gain of the multiplexed amplifier when connected to amplify a signal proportional to measured blue light will preferably be about ten times greater than its gain when so connected for red light, whereby for equal intensities of red, blue and green light, as well, the amplifier output will be about the same. However, the drifting in the amplifier output signals due to temperature, aging or other fatigue of the photosensor, respective amplifier input channels, and/or respective feedback or other color circuits or channels will normally be different for each color; for example, there may be a maximum of say three percent drift experienced in the red color channel, including the photosensor, when measuring red light, between the time the colorimeter is first turned on and a long time, say several hours, later, and similar maximum drifting of say five percent for the green color channel and 1.6 percent for the blue color channel. In the past each color channel would require periodically individual re-calibration to compensate for such drift even though with respect to the mentioned time parameter, for example, for each of the channels the fractional amounts of the maximum possible drift will be approximately the same.